Rong-Huei Yi

A new pyrazino[2,3-g]quinoxaline carbazole derivative with a nonplanar donor–acceptor–donor (D-A-D) architecture exhibits high molar extinction coefficients, low bandgap, bright solution and solid-state emission behavior, along with positive solvatochromism. Yellow and white host–guest organic light-emitting diodes fabricated with this solution-processable emitter exhibit high external quantum efficiency and bright electroluminescence along with high device operational lifetime.

Abstract

A new pyrazino[2,3-g]quinoxaline carbazole derivative with branched alkyl peripheral chains PQC-12b with a donor–acceptor–donor nonplanar architecture is reported. PQC-12b exhibits high molar extinction coefficients, low bandgap, bright emission behavior in solution and solid-state, along with positive solvatochromism. PQC-12b is used in organic light-emitting diodes (OLEDs) either as sole emissive material or as a guest in different host materials. Amongst these different host–guest OLEDs, CBP-based device with the optimized concentration of PQC-12b shows maximum values for power efficacy (PE_max) of 14.1 lm W⁻¹, current efficacy of 26.8 cd A⁻¹, external quantum efficiency (EQE_max) of 8.0%, and lifetime of 5.1 h at an initial luminance of 5000 cd m⁻² with a low power efficacy roll-off and roll-up character in current efficacy and EQE. A triplet exciton harvesting approach is employed by incorporating thermally activated delayed fluorescent emitter 4CzIPN to obtain a PE_max of 33.4 lm W⁻¹, a CE_max of 56.3 cd A⁻¹, an EQE_max of 15.3%, and a maximum luminance (L _max) of 29 400 cd m⁻². Utility of the compound in solution-processable white OLED is explored and interestingly showed a PE_max of 42.2 lm W⁻¹, a CE_max of 40.3 cd A⁻¹, an EQE_max of 12.0%, and L _max of 24 667 cd m⁻².

Applied Physics Letters, Volume 120, Issue 17, April 2022.
Recently, organic light-emitting diodes (OLEDs) are becoming increasingly attractive to information security, wearable healthcare, and other fields. These fields propose different requirements for performances of OLEDs, especially for voltage-controlled color tunability. In this study, it is proposed to use an ultrathin layer consisting of thermally activated delayed fluorescence (TADF) material as an emitting layer of OLEDs. On the one hand, compared to devices with an ultrathin phosphorescent emitting layer, the OLEDs with TADF show observable color-tunability. On the other hand, the color-tunable OLEDs with TADF show much higher efficiency than the color-tunable fluorescent OLEDs. It demonstrates that the reverse intersystem crossing process not only enhances the exciton utilization efficiency but also leads to an insufficient host-guest energy transfer. With this strategy, a color-tunable OLED is achieved with an external quantum efficiency about 8% and shows color variations over (0.04, 0.08) when its bias voltage increases from 4 to 8 V. By combining with a patterned mask technique, the color-tunable OLEDs can potentially be applied to the field of anti-counterfeiting and status lighting.

Terminal substituents and conformational changes have a great impact on OLED device efficiency and stability investigated by in situ Raman spectroscopy. Compared to BN-PhOH with the lowest EQE_max of 19.0 % and LT₅₀ of 1.7 h, BN-PhN(CH₃)₂ exhibited the longest OLED device lifetime (7.7 h), enhanced EQE_max of 24.1 %, and alleviated efficiency roll-off, as a manifestation of less conformational changes.

Abstract

There is little investigation into the impact of molecular conformation on device efficiency and degradation of boron-nitrogen thermally activated delayed fluorescence emitters (BN-TADF). Herein, three highly-efficient green BN-TADF emitters have been designed to unveil the impact of peripheral phenyl groups on device efficiencies and lifetimes. Compared to BN-PhOH with the lowest EQE_max of 19 %, BN-PhOCH₃ and BN-PhN(CH₃)₂ have achieved strongly enhanced EQE_max of 25.6 % and 24.1 %, respectively. Importantly, the device lifetimes (LT₅₀) are dramatically improved from 1.7 h of BN-PhOH to 4.4 h of BN-PhOCH₃ and 7.7 h of BN-PhN(CH₃)₂ without encapsulation. According to in situ Raman spectroscopy and simulations, BN-PhN(CH₃)₂ of less conformation change after aging exhibits the best photostability. It is proposed that the torsion angle change between the BN core and the peripheral phenyl group results in BN-TADF degradation. This knowledge means precisely tuning peripheral groups of BN-TADF can achieve both higher device efficiencies and longer lifetimes.

Publication date: July 2022

Source: Dyes and Pigments, Volume 203

Author(s): Hsiang-Ling Shen, Pei-Wan Hsiao, Rong-Huei Yi, Yi-Hua Su, Yin Chen, Chin-Wei Lu, Hai-Ching Su

The locally-excited triplet state (³LE) is found as an essential intermediate state in thermally activated delayed fluorescence (TADF) of compact electron-donor-acceptor system. ³LE and charge-separated states (¹CS and ³CS) are close to each other in energy, the interaction of three states causes TADF, which is analyzed by femtosecond/nanosecond transient absorption spectra and time-resolved electron paramagnetic resonance spectroscopy.

Abstract

We prepared an orthogonal compact electron-donor (phenoxazine, PXZ)-acceptor (naphthalimide, NI) dyad (NI-PXZ), to study the photophysics of the thermally-activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 μs (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non-negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02∼2.72 ps), the majority of the singlet CS state is short-lived, especially in polar solvents (τ _CR = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n-hexane). Nanosecond transient absorption spectroscopy detects a long-lived transient species in n-hexane, which is with a mixed triplet local excited state (³LE) and charge separated state (³CS), the lifetime is 15.4 μs. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat ³CS state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged ³LE/³CT states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native ³NI state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely-lying ³LE state is crucial for occurrence of TADF and this ³LE state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems.

Publication date: July 2022

Source: Dyes and Pigments, Volume 203

Author(s): Ji Ae Kang, Seung Chan Kim, Jun Yeob Lee

The photoluminescence quantum efficiency of a CaO:Eu NIR phosphor is significantly improved and stabilized at high temperature. By utilizing GeO₂ decomposition, the oxygen vacancies in the CaO lattice are effectively repaired. A record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125 °C.

Abstract

Near-infrared (NIR) luminescence materials with broadband emissions are necessary for the development of light-emitting diodes (LEDs) based light sources. However, most known NIR-emitting materials are limited by their low external quantum efficiency. This work demonstrates how the photoluminescence quantum efficiency of europium-activated calcium oxide (CaO:Eu) NIR phosphor can be significantly improved and stabilized at operating temperatures of LEDs. A carbon paper wrapping technology is innovatively developed and used during the solid-state sintering to promote the reduction of Eu³⁺ into Eu²⁺. In parallel, the oxygen vacancies in the CaO lattice are repaired utilizing GeO₂ decomposition. Through this process, a record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125 °C. The as-fabricated NIR-LEDs reach record photoelectric efficiency (100 mA@23.4%) and output power (100 mA @ 319.5 mW). This discovery of high-performance phosphors will open new research avenues for broadband NIR LED light sources in a variety of photonics applications.

A flexible organic light-emitting diode (OLED) using silk fibroin (SF) as a substrate exhibits a superior light extraction efficiency of up to 39.3%, which is nearly twice that of conventional glass. The naturally sourced SF film is a favorable candidate as a substrate for the fabrication of green flexible OLEDs with ultrahigh luminous efficiencies.

Abstract

It is an essential issue for the efficiency of flexible organic light-emitting diodes (OLEDs) to be improved in display and lighting applications. Herein, the use of silk fibroin (SF) as a flexible OLED substrate with enhanced light extraction efficiency (LEE) is reported. Regenerated SF is prepared via a casting method and then coated with conductive indium tin oxide (ITO) via magnetron sputtering. SF-based flexible OLED outperforms its reference device in terms of turn-on voltage, peak brightness, and LEE, where glass and polyethylene terephthalate are used as rigid and flexible substrates, respectively. The improved device performance can be ascribed to the low refractive index of SF and its lower surface roughness and more matched energy levels compared to those of other substrates, resulting in a high LEE. Notably, a molecular dipole layer can be formed between the interface of ITO and SF. The dipole moment pointing to the ITO surface effectively changes the ITO work function and reduces the potential barrier height for hole transfer. The work provides new ideas for constructing high-performance OLEDs on biomass-based substrates with excellent flexibilities and high LEEs.

Publication date: 15 August 2022

Source: Chemical Engineering Journal, Volume 442, Part 1

Author(s): Chia-Hsun Chen, Shih-Chun Lin, Bo-Yen Lin, Che-Yu Li, Yu-Cheng Kong, Yi-Sheng Chen, Shao-Cheng Fang, Ching-Huang Chiu, Jiun-Haw Lee, Ken-Tsung Wong, Chi-Feng Lin, Wen-Yi Hung, Tien-Lung Chiu

2,3-Disubstituted fluorene platforms for the construction of high-efficiency host materials for green phosphorescent organic light-emitting diodes.

Abstract

Fluorene is a classic three-membered polycyclic aromatic hydrocarbon, and it has been widely used in optoelectronic devices. Here we explore a simple and efficient strategy for the derivatization at the 2- and 3- positions in fluorene unit. By introducing different types of substituents, we design two pairs of 2,3-disubstituted fluorene isomers and use them as host materials for phosphorescent organic light-emitting diodes (PHOLEDs). The green PHOLEDs hosted by these fluorene derivatives realize high external quantum efficiencies (EQE) over 20 % with low efficiency roll-off. Particularly, the devices hosted by 2TRz3TPA and 2TPA3TRz achieve nearly 24 % EQE and 104 lm W⁻¹ power efficiency. These results clearly demonstrate that the 2,3-disubstituted fluorene platforms are potentially useful for constructing host materials.

High steric-hindrance windmill-type molecules with different donors and conjugation lengths emitting from UV to pure-blue are designed. The doped devices of these dyes exhibit excellent electroluminescence performances by harvesting triplet excitons via the hybridized local and charge-transfer excited state. An efficient UV device is achieved, providing an external quantum efficiency of 7.9% and CIE_y of 0.04.

Abstract

Hybridized local and charge-transfer (HLCT) excited-state compounds that enable full exciton utilization through a reverse intersystem conversion (RISC) from a high-lying triplet to a singlet state have attracted attention. Developing high-performance ultraviolet (UV) and blue organic light-emitting diodes (OLEDs) is challenging due to difficulties acquiring HLCT molecules with a large energy bandgap and a high photoluminescence. Herein, a new strategy for excellent-performance UV to blue emitters based on high steric-hindrance windmill-type structure is proposed. These emitters exhibit good thermal, morphological, and electrochemical stabilities, as well as HLCT excited-state characteristics. Results suggest that OLED using CTPPI efficiently emits UV light (396 nm, CIE_x,y = 0.16, 0.04) with a maximum external quantum efficiency (EQE) of 7.9% and currently ranks third in UV OLEDs. The lights of these devices are well modulated from UV/deep-blue to pure-blue with EQEs greater than 5% by regulating the locally excited (LE) and charge-transfer (CT) components. Experimental and theoretical investigations indicate that harvesting triplet excitons afford high electroluminescence efficiencies via HLCT excited states in the devices. This study provides an efficient strategy to achieve high-performance UV and blue OLEDs and offers great flexibility for material design.

White organic light-emitting diodes are developed using highly efficient and long range charge transfer complex emission between two blue phosphorescent emitters induced by an electric field.

Abstract

Herein, the authors report on charge-transfer (CT) complex emission-based organic light-emitting diodes (OLEDs), which emit light by electric field-induced intermolecular CT complex formation in the light-emitting layer. Two phosphorescent materials are chosen to have energy level offset for the CT complex formation. This process generates the CT complex, which efficiently emits only in the electroluminescent device by electric-field-induced complex formation. A phenylpyridine-ligand-based Ir compound and phenylimidazole-based Ir compounds are selected as the CT complex-forming materials, and the CT complex exhibits yellow emission under an electric field. Moreover, the doping of the two compounds in the host material produces white OLEDs, exhibiting both blue emission and yellow emission at a quantum efficiency of 13.7%. In particular, the color of the CT complex device can be tuned by the content of each material in the emitting layer. As an initial result, the quantum efficiency is acceptable, and further material development would enhance the quantum efficiency of the CT complex OLEDs. This light-emitting device can be used in various applications, e.g., a mono-color or a white-color light-emitting diode.

Publication date: 15 August 2022

Source: Chemical Engineering Journal, Volume 442, Part 1

Author(s): Zhan Yang, Xiangyu Ge, Wenlang Li, Zhu Mao, Xiaojie Chen, Chao Xu, Feng Long Gu, Yi Zhang, Juan Zhao, Zhenguo Chi

Publication date: June 2022

Source: Dyes and Pigments, Volume 202

Author(s): Jifu Sun, Mingmei Shu, Ningyuan Wang, Qun Wang, Huaiman Cao, Xue Zhang, Bo Wang, Jianzhang Zhao

A newly synthesized thermally activated delayed fluorescence emitter named DPPZ-DMAC is used as an ideal orange-yellow dopant featuring near-zero singlet−triplet splitting of 0.01 eV and high photoluminescence quantum yield of 91.6%. High-performance white organic light-emitting diode is achieved with an external quantum efficiency of 30.1%, power efficiency of 80.2 lm W^–1, and correlated color temperature of ≈3600 K.

Abstract

White organic light-emitting diodes (WOLEDs) based on thermally activated delayed fluorescence (TADF) emitters attract considerable attention owing to the advantages of full exciton harvesting, low cost, and environmental sustainability. However, compared to phosphorescent counterparts, the power efficiencies of TADF-based WOLEDs lag behind. Herein, a newly synthesized TADF emitter named DPPZ-DMAC featuring near-zero singlet−triplet splitting and high photoluminescence quantum yield (91.6%) is utilized as an ideal orange-yellow dopant for realizing a power-efficient WOLED with a mixed system consisting of a blue TADF sensitizer and a conventional fluorescent host. The balanced carrier transport, modulated energy transfer and exciton harvesting, and less exciton quenching can be realized simultaneously. Eventually, a warm WOLED is achieved with an external quantum efficiency of ≈30%, power efficiency of >80 lm W^-1, turn-on voltage of 2.5 V, and correlated color temperature of 3600 K. Moreover, the color coordinate of the resulting white emission is close to the standard blackbody radiation and can be precisely tuned inside the specific quadrangles given by the American National Standard Institute (ANSI), revealing a large prospect for indoor health lightings.